The first ionization energy of Ar is less than that of Ne. An explanation of this fact is that :

Options

A. The effective nuclear charge experienced by a valence electron in Ar is much larger than that in Ne
B. The effective nuclear charge experienced by a valence electron in Ar is much smaller than that in Ne
C. The atomic radius of Ar is larger than that of Ne
D. The atomic radius of Ar is smaller than that of Ne

Explanation

To explain why the first ionization energy of Argon (Ar) is less than that of Neon (Ne), we can break down the reasoning into several steps:

### Step-by-Step Solution:

1. **Definition of Ionization Energy**:
- Ionization energy is defined as the minimum amount of energy required to remove an electron from an isolated gaseous atom.

2. **Noble Gases and Their Stability**:
- Both Neon (Ne) and Argon (Ar) are noble gases, which means they have a stable electronic configuration. This stability generally leads to high ionization energies.

3. **Position in the Periodic Table**:
- Neon has an atomic number of 10, while Argon has an atomic number of 18. Argon is located below Neon in Group 18 of the periodic table.

4. **Atomic Size**:
- As we move down a group in the periodic table, the atomic size increases. This is due to the addition of new principal energy levels (shells). For Argon, there are more electron shells compared to Neon.

5. **Distance from Nucleus**:
- The increased atomic size means that the valence electrons in Argon are further away from the nucleus compared to those in Neon. This increased distance reduces the electrostatic attraction between the nucleus and the valence electrons.

6. **Shielding Effect**:
- With the addition of new electron shells in Argon, there are more inner-shell electrons that shield the valence electrons from the full effect of the nuclear charge. This is known as the shielding or screening effect, which further decreases the effective nuclear charge felt by the valence electrons.

7. **Effective Nuclear Charge**:
- Although the nuclear charge (number of protons in the nucleus) increases as we move from Neon to Argon, the combined effects of increased atomic size and shielding more than compensate for this increase. Thus, the effective nuclear charge experienced by the valence electrons in Argon is less than that in Neon.

8. **Conclusion**:
- As a result of the increased atomic size and the shielding effect, the valence electrons in Argon are held less tightly by the nucleus compared to those in Neon. Therefore, the first ionization energy of Argon is less than that of Neon.